This document discusses the persistence and survival mechanisms of weeds. It begins by defining persistence as the continued existence of something and outlines the objectives of understanding what makes some plants able to grow where unwanted and why they are difficult to control. The main survival mechanisms of weeds discussed are reproduction, plasticity, and mimicry. Weed reproduction includes seed production, dispersal, germination, dormancy and longevity. Plasticity allows weeds to tolerate a wide range of conditions. Mimicry allows weeds to hide in crop seeds or mimic crop morphology. The document provides details on each of these mechanisms.

1. 6/7/2017
1
Weed biology
Dr. Kudra, A. B
Practicals
• What practical?
Persistence and survival
mechanism of weeds
.. ……
Persistence
• Firm or obstinate continuance in a course of
action in spite of difficulty or opposition.
• The continued or prolonged existence of
something.

2. 6/7/2017
2
mechanisms
• Nature of life and Biology
5. Persistence and survival
mechanism of weeds
Objectives
i. What is it that makes some plants
capable of growing where they are not
desired?
ii. Why are they difficult to control?
iii. What are their modes of survival?
Persistence and survival mechanismof weeds
• Weed persistence is a measure of the adaptive potential
of weeds that enables them to survive in disturbed
environment such as i. Crop land ii. Recreational site iii.
Irrigation canal and iv. Pastures
• Persistence of weed species is largely influenced by
climatic, edaphic and biotic factors
Main adaptive/survival mechanisms
• Weed reproduction
• Plasticity
• Mimicry

3. 6/7/2017
3
1. Weed reproduction
• Seed production
• Seed dispersal
• Seed germination
Seed production
1. Annual weeds-Seed production
• Survival of many flowering plants
depends on production of a sufficient
number of viable seeds
• Annual weeds produce large quantities
of seed
• Production of abundant and small seeds
ensures a high probability of dispersal
and reinfestation
…
• A single plant of annual weed can produce enough
seeds in one season to cover an area of one acre
next year
• Some have ability to germinate and produce seed
before crop
• Many can produce seeds even after been cut off
Seed production
Weed Number of seeds per plant
Echinochloa crus-galli 7,160
Chenopodium spp 72,450
Cyperus esculentus 2,420
Amaranthus spinosus 200,000
A.retroflexus 117,400
Zea mays 700+
Striga 40,000 – 50,000+
Implication; one weed plant causes potential problem
in next year’s crop

4. 6/7/2017
4
Survival....
Perennial weeds- Perennating organs
• Dual modes of reproduction
• Stolons or creeping above-ground
• Rhizomes
• Bulbs and aerial bulblets
• Tubers
Survival….
• Storage organs may act as 'perennating organs‘
These are used by plants to survive adverse periods
in the plant's life-cycle
• During these periods, parts of the plant die and
then when conditions become favourable again, re-
growth occurs from buds in the perennating organs
..
• Their ability to produce both vegetativeand seeds
make them very competitive and aggressive in
cropping system
• Even if the aerial parts die, vegetative continue to
live and send up new growth
Survival.……
Seed bank
Is the primary source of new infestations of weeds each year
The components
• Seed rain- a process where by seeds produced by a weed
plant are returning back to the soil
• Some remain on the soil surface, most of them enter soil
profile due to no. of operations or natural movement eg
Avena spp attracted towards moist areas

5. 6/7/2017
5
Seed production……
• Seed rain replenishes reservoir of viable seeds in
the soil
• Soil seed reservoir commonly called SEEDBANK is
seed produced in given area + weed seeds brought
by various agents of seed dissemination
• Expressed in g/gram of soil
• The size of seedbank in agricultural land ranges
from zero to as much as 1 million seed/m2
Seed dispersal
a. Natural dispersal
• Wind
• Water
• Animals Boerhavia
• Forceful dehiscence
b. Artificial dispersal: basically a result of
man’s activities
• Machinery, Livestock feed, hay and straw,
manure
Dispersal...
Adaptation for dissemination
• Many weed seeds have some structure
that gives seed buoyancy in air
• Ability to attach to something such as
agriculture tools, animals skin etc
• By knowing how weeds are disseminated
one may be able to take steps to reduce
or avoid further spread
• Bidens
pilosa
Dispersal…
Consequences of weed dispersal
• Expanding their territory e.g chromoleana
odoratain Mara, invaded number of acres within
short time

6. 6/7/2017
6
Seed germination
• What is really concern to weed managers is not
the fact that seed germinate, but the fact that
they do not germinate because they are dormant
• Weeds use this mechanism to survive between
cropping seasons
Germination …
Seed dormancy
• Is a state in which viable seed fails to germinate
under favourable conditions (moisture, temp &
oxyegen)
• To escape unfavorable environment conditions
• Dormancy is very important for seed longevity, &
efficientsurvival mechanism of weeds
Germination..
• Weed seed longevity – viability over time
• Varies by species as;
• eg. Foxtail -20 years
• Johnsongrass - 20 years
• Redroot pigweed -40 years
• Striga -20 years
• Implication; weed seed production even for one
year can assure that weeds will be a problem for
many years
germination..
• Typesof seed dormancy
a. Innate dormancy (primary dormancy)
b. Secondary dormancy
a. Innate dormancy
• Viable seed fail to germinate in presence of
favourable conditions (primary dormancy)

7. 6/7/2017
7
germination..
• Factors for primarydormancy
• Immature Embryo
• Physical restriction associated with impermeable
seed coat
• Presence of chemical inhibitors – abscisic acid
germination..
• Secondarydormancy
i. Induced dormancy
• Inability of a mature viable seed to germinate due
to being exposed to unfavourable conditions, &
persists after environmental conditions change
ii. Enforced dormancy
• When this external limitation is removed, the seed
germinates
2. Environmentalplasticity
• Capable of tolerating and growing under a
wide range of climatic and edaphic conditions
• Weed seeds resist decay for long periods in
soil and remain dormant.
3. Mimicry
• Another mechanism weeds use to survive
between cropping seasons is mimicry

8. 6/7/2017
8
mimicry..
5. Mimicry
• Hiding in crop seed to be planted the
next year/ morphology escape from
weeding
• Weeds often produce seed that is the
same size and shape as crop seed
mimicry..
Typesof mimicry
• Seed mimicry
• E.g. red rice seeds by cultivated rice
• Vegetativemimicry
• E.g. wild rice and cultivated rice
• Biochemicalmimicry
• Developing resistance to herbicides previously
used, eg. Amaranthusspp vs atrazine in maize
Task
• Assignment
• A group of 4 students
• Write down the survival mechanisms of Parthenium
hysterophorus(hints; Introduction,Distributionin
Tanzania, harmful effects caused, Survival
mechanisms)
• Due 18th May 2017
Weed ecology
What is weed ecology
Factors determines the distribution, prevalence,
competitionability, bahaviour and survival of
weeds

9. 6/7/2017
9
Crop – Weed- environmentinteractions
i. To understand weed-environment interactions
ii. To be able to define plant competition and to
know what resources plants compete for
iii. To understand the characteristicsthat make a
plant competitive
Interactions..
weed-environmentinteractions
• Factors that determine a weed’s ecological
interactionsare
• Climatic, edaphic and biological
• Light intensity and duration influence growth,
reproduction and distribution of weeds
• Temperature – affect survival and distribution of
weeds
Interactions…
• Yellow nutsedge does well in the subhumid tropics
and warm, temperate regions
• Waterhyacinth,an important aquatic weed in the
tropics and subtropics
• Soil/edaphic – some weeds species are alkali plants
known as basophiles (pH 7.4 – 8.5) eg. quackgrass
Interactions…
• Inhabit in acidic soil termed as acedophile (pH 4.5 –
6.5) eg. Cynodondactylon, Digitaria sanguinalis
• Neutrophile (6.5 – 7.4), saline soils
• These weeds serves as indicator of different soil
reactions
• A shift in soil pH could also cause a shift in weed
spectrum eg. continuous use of ammonium
sulphate fertilizers

10. 6/7/2017
10
Interaction..
• Some weeds species e.g. Commelina benghalensis
thrive in moist soil conditions
Climate change
CO2 and Plant/Weed Biology: Overall.
C3 vs C4 weed/crop
• Evidence to date indicates rising CO2 per se has
effect on:
– weed populations.
– crop losses due to weeds.
– species diversity
– weed control efforts.
As carbon dioxide increases,glyphosate efficacy is reduced
Ziska et al. Weed Science 52:584-588,2004
WeedBiomass(gm-2
)
0
100
200
300
400
500
600
700
Broadleaf weeds(C3) Broadleaf weeds(C4) Grassy weeds (C4)
ElevatedCO2
**
p= 0.07
AmbientCO2
2004
Not Sprayed
Broadleaf weeds(C3) Broadleaf weeds (C4) Grassy weeds (C4)
Elevated CO2
0
10
20
30
40
50
60
**
Ambient CO2
2004
Sprayed
WeedBiomass(gm-2)
Interaction …
Weed crop interaction
• crops and weeds may interact either in leaves
and/or roots

11. 6/7/2017
11
Interactions..
Weed and crop interaction
• crops and weeds may interact either in leaves and/or roots
Some interactions are:
• Competition
• Allelopathy
• Symbiosis
• Amensalism- e.g. plant grow under shadow
• Commensalism- one benefit, other is not affected (epiphytes)
• Parasitism
Interactions..
Plant competition
• Latin competere,which means to ask for the same
thing another does
• competitionoccurs when two or more plants seek
what they need and the immediate supply is below
combined demand
• The competitionis real; it is for life
Interactions..
• It affects the growth of plants
• Reduction in crop growth and yield due to weeds
results from competition for nutrients, moisture
and sunlight
• Competition is what weed control is about
Interactions..
Factors that control the degree of competition
• Density,
• Distribution, (spacing between plants)
• Duration or how long weeds are present is
important
• General rule - for every unit of weeds grown, there
will be one less unit of crop grown

12. 6/7/2017
12
..
• If weeds emerge after the crop is well established,
they may not pose as serious problem as those that
emerge before the crop plants emerge and
established
• Competition between weeds and crops is more
severe when the competing plants have similar
vegetativehabits and demands upon resources
Interaction..
• Competition between plants is maximum when
available resources for crop growth become
limiting.
Interactions..
Forms/Natureof plant competition
• Competition between plants of the same species
• Competition between plants of different species
Typesof competition
1. Above ground competition
2. Below ground competition (roots)
Competition……
Aboveground
• Competitionfor light
• Whenever a leaf is shaded by another, there is
competitionfor light
Factors affecting competitionfor light
• LAI-Plantswith large leaf area indices (LAI) have a
competitiveadvantage with plants with smaller
leaf areas

13. 6/7/2017
13
Competition……
Leaf inclinationand arrangement
• Plants with opposite leaves are probably less
competitivethan those with alternate leaves
Plant height
• Plants that are tall or erect have a competitive
advantage for light over short, prostrate plants
• The effect of shading is independent of direct
competitionfor water or nutrients.
Weed-crop..
Interaction
• Horizontal leaves vs upright leaves
Competition……
Species
• monocots are shade sensitive than dicots means do
not perform well when shaded
• This concept is used in intercropping eg. Cereals
and legumes

14. 6/7/2017
14
Competition……
Factors for which plantsdo not compete
• Space
• Oxygen
• Carbon dioxide
• Agents of pollination
Note (space) plant germinate first and grow faster
tend to exclude others
Interaction….
Below ground competition
• Competitionfor nutrients
• Nitrogen, phosphorus, and potassium
• In general, weeds are more efficient and will
absorb as much or more than crops
• Maximum benefits from fertilizer are usually under
minimum number of weeds
The nutrient content of weed-free and maize-
free weeds shows that weeds contained more of
each of the nutrient
• ..
Meanpercentcomposition
Plant N P K Ca Mg
Maize 1.2 0.2 1.2 0.2 0.2
Galinsoga 2.7 0.3 4.8 2.4 0.5
Pigweeds 2.6 0.4 3.9 1.6 0.4
Source:Zimdahl,2007
Interaction….
Competitionfor water
• It is probably the most critical of all plant growth
requirements
• Usually measured by a phenomenon known as
Water Use Efficiency (WUE)
• WUE is defined as the amount of water required to
produce a unit of dry matter
• weeds have high water use efficiency compared to crops

15. 6/7/2017
15
Interaction….
Factors determine below ground competition
• Root zone
• If crop and weeds occupy the same root zone
competitionfor water tends to much stronger
• More competitive plants have faster-growing, large
root systems so they are able to exploit a large
volume of soil quickly
Interaction….
Species
• Species which adapted to grow in abundant soil
moisture usually have a lower WUE than species
adapted under low soil moisture
• Cyperus vs Striga
Interaction….
Critical Period of Weed Competition(CPWC)
• When does competition begin?
• How long does competition last?
• A. The first study asks what the effect is when the crop is
kept weed-free from emergence for certain periods of time
and then weeds are allowed to grow for the rest of the
growing season (weed free period)
• Eg. For the first 2, 4, 6, ….10 weeks
Interaction….
• B. The second kind of study asks what the effect is when
weeds emerge with the crop and are allowed to grow for
defined periods of time. After each of these times, the crop
is then kept weed-free for the rest of its growing period
(weed infested period)
• eg. For the first 2, 4, 6, ……10 weeks

16. 6/7/2017
16
Interaction….
• Weed free
• Graphs
• Weed infested
Weed-crop..
Interaction….
• i.e. It is the period of time between that period
after planting when weed competition does not
reduce yield and the time period after which weed
presence does not reduce yield
• Critical period of control (or critical period of weed
competition):It is the shortest period during which
competitionis at maximum
Interaction….
• NB: the critical competition is not uniform to all
crops, for all weed and to all geographical regions

17. 6/7/2017
17
Interaction….
Allelopathy
• Is the interaction, usually detrimental (the pathos),
of one plant on another by toxic chemical
substances from living plant parts through their
release when a plant dies or their production from
decaying tissue
Allelopathy..
• Potentialof allelopathy in weed management
Practical
Weed identification (done)
Weed surveys/ infestation assessment
• Species
• Weed density
• Frequency
• Abundance
Seed bank
Field Survey
• Examine and record the area and features of an
area of land so as to construct a map, plan, or
description

18. 6/7/2017
18
Why weed survey
• To pinpoint major weed problems/species that
require attention/ most serious weed in growing
area
• Showing distribution and severity of infestations
(map)
• Direct research toward new or improved control
measures
Procedures
• Select no. of fields in selected survey area, example
(5 – 10) different fields
• Record GPS values of the selected fields
• Survey a field by using 1m x 1m quadrate
• In a zigzag/ diagonal establish at least 5 -20
quadrates in each field
Procedure…
• Identify, counts and record all weeds in each
quadrate (species, broadleaves, grasses, sedges,
annual, perennial etc)
• Then, summarize your data using graphs and/or
Tables/ map. The summary should include
• Species, morphology, life span, Frequency (F),
Uniformity (U), Density (D), Mean field density
(MFD) and Relative abundance (RA)
How will you determine F, U, D, and MFD
• F = No. of fields in which species occurred divide
by total no. of fields surveyed x 100 (%)
• U = No. of quadrates per field in which species
occurred divide by total no. of quadrates x 100
• Density = no. of individual species/metersquare
• MFD = totaling each field density of a species
divide by total no. of fields

19. 6/7/2017
19
Relative abundance (RA)
• RA = RF + RU + RD whereby;
• RF = frequency value of species (k) divide by sum of
frequency values for all species x 100
• RU = field uniformity value of species (k) divide by
sum of field uniformity for all species x 100
• RD = mean field density value of species (k) divide
by sum of mean density values for all species x 100
Interpretation
• The weeds which appeared with highest
frequencies, field uniformities and mean field
densities, indicating that these weeds are the more
difficult to control.
• So, these species should be carefully monitored.
Interpretation..
• All types of weeds that have field frequencies <50%,
field uniformities <35% and mean field densities <2
plants m2 may either less competitivewith a crop or
may be effectively controlled by current weed
managementpractice in the studied area.
Interp..
• Relative abundance provides an indication of the
overall weed problem posed by a species.
• The higher RA value indicates the most serious
weed in growing areas

20. 6/7/2017
20
Practical work
In a group of 5
• Conduct weed survey at crop museum
• Select at least 5 different fields growing crops
• Establish at least 5 quadrates per field (in a
zigzag/diagonal)
• Collect all the information as per our discussion
• Summarize your data as per today’s lecture
• Option: Map showing distribution and severity of
infestations (due: 21 June 2017)
Test 2
15 or 22 June 2017
The early the better